In certain types of offshore drilling, a string of casing or riser will be connected between a subsea wellhead assembly at the sea floor and a surface wellhead at a platform located at the surface. For example, with a tension leg platform, which is a buoyant platform supported with tendons held under tension, a subsea wellhead will be installed on the sea floor at the upper end of a string of conductor. A string of casing will extend into the well and have a tieback mechanism on its upper end.
An outer or low pressure riser will be secured to the subsea wellhead and extend upward to the platform. A surface wellhead will be installed at a lower deck level on the platform, approximately 90 feet below the drilling rig floor. A blowout preventer and drilling riser will connect to the surface wellhead and extend upward to the rig floor. Then, a tieback string of casing serving as a high pressure inner riser will be lowered from the platform through the blowout preventer and surface wellhead and latched into the tieback mechanism at the subsea wellhead. The operator applies tension to the tieback string and adjusts a load shoulder at the surface wellhead for maintaining the tieback string in tension. The operator then completes drilling of the well through the high pressure riser string.
A number of different systems have been used and proposed in the past for maintaining the tieback string in tension. Some of these systems employ a locking member which will ratchet on a mandrel in one direction and support weight in the other direction to maintain the string in tension. While these systems are workable, improvements to reduce cost and facilitate installation are desirable.